netdev - Re: [PATCH v5 net-next 00/15] af

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Message-ID: <916e8bdd54700b2585f4556dff4f96f8dbbf8092.camel@redhat.com>
Date: Fri, 29 Mar 2024 10:55:29 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>, "David S. Miller"
	 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	 <kuba@...nel.org>
Cc: Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH v5 net-next 00/15] af_unix: Rework GC.

On Mon, 2024-03-25 at 13:24 -0700, Kuniyuki Iwashima wrote:
> When we pass a file descriptor to an AF_UNIX socket via SCM_RIGTHS,
> the underlying struct file of the inflight fd gets its refcount bumped.
> If the fd is of an AF_UNIX socket, we need to track it in case it forms
> cyclic references.
> 
> Let's say we send a fd of AF_UNIX socket A to B and vice versa and
> close() both sockets.
> 
> When created, each socket's struct file initially has one reference.
> After the fd exchange, both refcounts are bumped up to 2.  Then, close()
> decreases both to 1.  From this point on, no one can touch the file/socket.
> 
> However, the struct file has one refcount and thus never calls the
> release() function of the AF_UNIX socket.
> 
> That's why we need to track all inflight AF_UNIX sockets and run garbage
> collection.
> 
> This series replaces the current GC implementation that locks each inflight
> socket's receive queue and requires trickiness in other places.
> 
> The new GC does not lock each socket's queue to minimise its effect and
> tries to be lightweight if there is no cyclic reference or no update in
> the shape of the inflight fd graph.
> 
> The new implementation is based on Tarjan's Strongly Connected Components
> algorithm, and we will consider each inflight AF_UNIX socket as a vertex
> and its file descriptor as an edge in a directed graph.
> 
> For the details, please see each patch.
> 
>   patch 1  -  3 : Add struct to express inflight socket graphs
>   patch       4 : Optimse inflight fd counting
>   patch 5  -  6 : Group SCC possibly forming a cycle
>   patch 7  -  8 : Support embryo socket
>   patch 9  - 11 : Make GC lightweight
>   patch 12 - 13 : Detect dead cycle references
>   patch      14 : Replace GC algorithm
>   patch      15 : selftest
> 
> After this series is applied, we can remove the two ugly tricks for race,
> scm_fp_dup() in unix_attach_fds() and spin_lock dance in unix_peek_fds()
> as done in patch 14/15 of v1.
> 
> Also, we will add cond_resched_lock() in __unix_gc() and convert it to
> use a dedicated kthread instead of global system workqueue as suggested
> by Paolo in a v4 thread.
> 
> 
> Changes:
>   v5:
>     * Rebase on the latest net-next.git

I went through this and LGTM. Additionally, I think it would be better
merge this at the beginning of the cycle rather then later.

Acked-by:

Paolo Abeni <pabeni@...hat.com>